1. Field of the Invention
The present invention relates to a sheet conveying device, a sheet punching device, a sheet processing device, an image forming apparatus, and a sheet conveying method to be implemented in the sheet conveying device, the sheet processing device, and the image forming apparatus.
2. Description of the Related Art
In a typical sheet post-processing apparatus, a punching unit measures a position of a side edge of a sheet-like recording medium (hereinafter, “sheet”), and decides a punching position on the sheet based on the measured position of the side edge. This approach allows accurate setting of the punching position. Related technologies have been disclosed in, for example, Japanese Patent No. 3363725 and Japanese Patent Application Laid-open No. 2003-248410.
Japanese Patent No. 3363725 discloses a conventional punching device that measures a position of a side edge of a sheet and decides a punching position based on the measured position of the side edge. Specifically, the punching device includes a punching unit that is movable in a punching direction that is perpendicular to a sheet conveying direction and can punch a conveyed sheet at a desired position; a detecting unit that detects a side edge of a sheet conveyed to the punching unit in the sheet conveying direction; and a moving unit that moves the punching unit in the punching direction based on the position of the side edge detected by the detecting unit. In particular, the detecting unit is moved in advance to a position near the side edge of the conveyed sheet based on information about a size of the sheet.
However, with the current increase in the sheet conveying speed and the printing speed, the time available for moving the detecting unit to the side edge of the conveyed sheet is becoming shorter and shorter. Therefore, it is technically difficult to move the detecting unit in punching units with fast processing speed. To take care of this issue, a contact image sensor (CIS) is now a days used in image forming apparatuses for measuring a position of the side edge of a conveyed sheet. For example, Japanese Patent Application Laid-open No. 2003-248410 discloses a conventional technology in which a CIS is used to detect a side edge of a conveyed sheet. Specifically, the CIS is arranged in a sheet-conveying area such that reading pixels of the CIS are aligned in a direction substantially perpendicular to a sheet conveying direction. One-seventh of the total reading pixels are repeatedly read in a shorter period (TS) to detect a leading end of a conveyed sheet. After a predetermined waiting time has elapses from a timing of detection of the leading end of the sheet, image writing in a sub-scanning direction is started by irradiating a laser onto a photosensitive element. Meanwhile, six-seventh of the total reading pixels are read in a longer period to detect a lateral position of the conveyed sheet. A misregistration amount is calculated based on the detected lateral position, and a writing position in a main-scanning direction on the sheet is corrected based on the misregistration amount.
In this manner, a processing speed for detecting a side edge of a sheet can be improved by using the CIS. However, such an advantage can be achieved only when the CIS is mounted properly with good precision. Therefore, it is necessary to check, after mounting the CIS, whether the CIS has been mounted properly. One method of checking whether the CIS is mounted properly is as follows. That is, a sheet is set in an offset manner in a sheet feeding unit of an image forming apparatus, and an image is formed on the sheet on a trial basis to see whether image-misalignment occurs.
The above technique can be used if a CIS is mounted in an image forming apparatus right from the beginning, i.e., during assembly of the image forming apparatus. However, a CIS can be provided in a punching unit of a sheet post-processing apparatus, i.e., at a later stage of assembly of the image forming apparatus, for improvement of the accuracy of the punching position. Because a punching unit is often an optional device to be installed depending on a request from users, the punching unit is generally set and checked by a field service person at a customer location, so that assembly error or checking failure is likely to occur. The accuracy of the punching position also depends on how a sheet is conveyed. For example, if a sheet is conveyed without skew or lateral misregistration, the accuracy of the punching position will be better. Therefore, whether a CIS is operating normally, or a CIS has been mounted at all, cannot always be checked by printing an image on a sheet on a trial basis as in the above technique. In other words, the above technique is not always effective to determine whether a measuring unit for measuring a position of a side edge of a sheet is operating normally, or whether the measuring unit has been mounted at all in the apparatus.